The present invention relates to an optical transceiver which is provided at each terminal in a data communication system employing a pair of optical data buses which are of a linear bus type, such as an optical data bus for aircraft used to transmit optical signals in opposite directions.
In general, a linear bus system is one way of forming an optical data bus. In the linear bus system a plurality of terminals are connected to a plurality of optical couplers inserted in an optical data bus, and hence are linearly arranged and connected (without forming loops or stars). Since the optical coupler is directional, however, two optical data buses are needed for transmission and reception between terminals. Accordingly, each terminal has two systems of optical transceivers so that it is capable of transmission and reception over either of the two optical data buses.
A description will be given first, with reference to FIG. 1, of a conventional linear bus type optical data communication system. Each terminal equipment TEi (i=1, 2, . . . ) is connected via optical couplers C1 and C2 to a linear bus type first optical data bus L1 for transmission in a first direction A and a linear bus type second optical data bus L2 for transmission in a second direction B opposite to the first direction A. The terminal equipment TEi comprises an optical transceiver PTi connected to the optical couplers C1 and C2 via transmitting optical fibers FT1 and FT2 and receiving optical fibers FR1 and FR2, a bus controller BCi for controlling communications, and a data terminal DTi for transmitting and receiving data. The terminal equipment TEi provides an optical data signal on the optical data buses L1 and L2 and receives an optical data signal therefrom. Each terminal equipment TEi delivers the optical data signal, using a different time slot so that the optical data signal will not interfere with those from other terminal equipment. The bus controller BCi receives the data transmitted from the terminal equipment TEi so as to see if the transmitted and the received data do not coincide, and if not, the bus controller BCi will judge that an abnormality is present in one of the transmitting and receiving routes such as the optical fibers FT1, FT2, FR1 and FR2, and the optical couplers C1 and C2. The bus controller BCi, which controls communications between data terminals DTi according to predetermined rules, is connected to a data input/output end of each data terminal DTi. The optical transceiver PTi is connected between the bus controller BCi and the corresponding optical couplers C1 and C2. Transmission data SD input from the bus controller BCi into the optical transceiver PTi is supplied in parallel to transmitting circuits T1 and T2 and, after being modulated, they are supplied to light emitting elements (LED's, for example) LE1 and LE2, wherein they are converted to optical signals, which are provided to input ports pb of the optical couplers C1 and C2 via the optical fibers FT1 and FT2, respectively.
In each optical coupler C1 an optical data signal provided to its input port pa from the first optical data bus L1 or an optical data signal provided to the input port pb from the optical fiber FT1 is split into two optical data signals, which are provided to output ports pc and pd. The optical data signal at the output port pc is supplied via the first optical bus L1 to the neighboring optical coupler C1, whereas the optical data signal at the output port pd is supplied via the optical fiber FR1 to a photodetector (a photodiode, for example) PD1 of the terminal equipment TEi itself, wherein it is converted to an electric signal, which is received by a receiving circuit R1 and then demodulated.
In each optical coupler C2 an optical data signal provided to its input port pa from the second optical data bus L2 or an optical data signal provided to the input port pb from the optical fiber FT2 is split into two optical data signals, which are provided to output ports pc and pd. The optical data signal at the output port pc is transmitted via the second optical data bus L2 to the neighboring optical coupler C2, whereas the optical data signal at the output port pd is provided via the optical fiber FR2 to a photodetector PD2 of the optical transceiver PTi of the terminal equipment TEi itself, wherein it is converted into an electric signal, which is received by a receiving circuit R2 and then demodulated.
The outputs of the receiving circuits R1 and R2 are applied via an OR gate OR to the bus controller BCi, while at the same time they are applied to a comparator CP, wherein they are checked with each other, and if they do not coincide, an error flag EF is provided to the bus controller BCi. When the first transmitting and receiving system composed of the transmitting circuit T1, and light emitting element LE1, the optical fiber FT1, the optical coupler C1, the optical fiber FR1, the photodetector PD1 and the receiving circuit R1 and the second transmitting and receiving system composed of the transmitting circuit T2, the light emitting element LE2, the optical fiber FT1, the optical coupler C2, the optical fiber FR2, the photodetector PD2 and the receiving circuit R2 are both normal during transmission from the terminal equipment TEi, the error flag EF will not be set. While the terminal equipment TEi is not transmitting but instead is receiving optical data from another terminal equipment TEj (j.noteq.i) in a certain time slot via either one of the first and second optical data buses L1 and L2, no optical data is provided in that time slot of the other optical data bus. Consequently, the outputs of the receiving circuits R1 and R2 are checked with each other in the comparator CP and the error flag EF is provided, but since the terminal equipment TEi is not transmitting, the error flag EF is ignored in the bus controller BCi.
As described above, an abnormality in the transmitting and receiving systems of the optical transceiver including the optical couplers C1 and C2 inserted in the data buses can be detected by comparing the outputs of the receiving circuits R1 and R2 in the comparator CP.
The optical data communication shown in FIG. 1 uses the Manchester code, for example. The Manchester code represents logic "1" with "01" (i.e. "LH") and logic "0" with "10" (i.e. "HL"); so that even if either the same logic "1" or "0" is repeated many times, the Manchester coded codes therefor are repeated alternation of "L" and "H" in either case. That is, logical data is made alternating by the Manchester coding, accordingly the Manchester code is suitable for optical data communications. In the following description of the invention "one bit" indicates one Manchester code "01" or "10" and "one bit length" indicates its length. According to such Manchester coding, it is only when the bit "01" is followed by the bit "10" that the high level continues and the greatest number of high levels which can continue is two (one bit length). Similarly, the low level continues only when the bit "10" is followed by the bit "01" and the greatest number of low levels which can continue is two (one bit length). In the optical data communication utilizing such Manchester codes, a signal containing a sequence of logical levels which are impossible to occur in the Manchester coding, for example, a signal containing three of more high and low levels in succession (1.5 bits in Manchester code), is used as a synchronizing signal so that it is distinguishable from Manchester coded data. This prior art example is assumed to employ, as a synchronizing signal SYN, a signal "HHHLLL" (i.e. "111000") of a 3-bit length (in Manchester code equivalent).
As shown on Row A in FIG. 2, one frame of the signal which is used in such an optical data communication is composed of a 3 bit long synchronizing signal SYN, 16-bit Manchester coded data DD and a 1-bit parity bit PB. As mentioned above, the synchronizing signal SYN is composed of a high-level signal of an l.gtoreq.5 bit length and a low-level signal of an m.gtoreq.1.5 bit length. In the example of FIG. 1, l=m=1.5. As shown on Row B in FIG. 2, one word string is made up of a plurality of such frames and a plurality of such word strings are sequentially coupled, with a 4-bit, low-level guard gap GG interposed therebetween, thus forming a message. In the optical data communication such a message is inserted in an empty time slot for data transmission. Messages are each separated by a terminal gap TG composed of a low level of at least eight bits.
There has been a strong demand for an economical and small-sized optical data communication system and the same is true of the optical transceiver therefor. However, no satisfactory solutions have been proposed so far.